1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly, to an optical pickup device which uniformly shapes waveforms of a laser beam.
2. Discussion of the Related Art
A conventional optical pickup device for shaping waveforms will be described with reference to the accompanying drawings.
FIG. 1 is a schematic view illustrating an optical pickup device for shaping waveforms using a cylinder lens, according to the first embodiment of the prior art.
The conventional optical pickup device includes a laser diode 21, a collimation lens 22, a cylinder lens 23, a beam splitter 24, a reflecting mirror 25, an object lens 26, and a disk 27. The laser diode 21 emits an elliptical laser beam. The collimation lens 22 collimates the emitted laser beam to be incident on the cylinder lens 23.
The laser beam emitted from the laser diode 21 will be described with reference to FIGS. 2a and 2b.
FIG. 2a is a graph illustrating the distribution of laser beam emitted from a general laser diode. FIG. 2b is a graph illustrating a beam shape of equivalent distributed section of FIG. 2a.
As shown in FIGS. 2a and 2b, in general, the laser beam emitted from the laser diode 21 has an elliptical shape when it is taken along equivalent distributed section due to difference of emitting angle against horizontal (x) and vertical (y) direction.
In the conventional optical pickup device, the focus size of the laser beam formed on the disk 27 depends on distribution of intensity of incident light. If angle difference of the emitted laser beam is large, the focus size of the laser beam of which emitting angle is large becomes smaller while the focus size of the laser beam of which emitting angle is small becomes larger. As a result, unbalance of the beam size occurs in pit and track directions on the disk 27.
Therefore, it is necessary to shape waveforms so as to uniformly distribute the intensity of incident light in each direction.
The laser beam emitted to the cylinder lens 23 passes without being reflected in vertical direction and is widened as being reflected in horizontal direction.
In other words, waveforms of the elliptical laser beam emitted from the laser diode 21 can be shaped through the cylinder lens 23, which is a concave lens, to uniformly maintain a radiation angle in vertical direction and a radiation angle in horizontal direction.
The laser beam of which waveforms are shaped by the cylinder lens 23 is incident on the collimation lens 22. Then, the path of the laser beam is changed to parallel direction to have the laser beam be incident on the beam splitter 24. The laser beam is then reflected by the reflecting mirror 25 and focused on the disk 27 by means of the object lens 26.
FIGS. 3a and 3b are schematic views illustrating an optical pickup device for shaping waveforms using a prism and two prisms, respectively, according to the second embodiment of the prior art.
In this embodiment of the prior art, a prism 31 or two prisms instead of the cylinder lens 23 of FIG. 1 can be used to shape waveforms of the elliptical laser beam emitted from the laser diode 21 uniformly in horizontal and vertical directions.
The aforementioned optical pickup device according to the prior art has several problems.
Since the cylinder lens and one prism or two prisms are used to shape waveforms of the laser beam, the device becomes complicated mechanically, which is thus not economical.